It is known that nickeliferous oxide ores, e.g., those referred to as laterites, comprising limonits and saprolite, are the world's largest potential sources of nickel and cobalt.
The ability to beneficiate these ores by conventional techniques has placed these ores at an economic disadvantage in that these ores cannot be concentrated by magnetic separation by froth flotation as compared to nickeliferous sulfide ores which can be easily concentrated to substantially high levels of nickel by well known methods, such as froth flotation and matte smelting.
One process for recovering nickel and cobalt is the well known Moa Bay process involving acid leaching at elevated temperatures and pressures at which iron oxide and aluminum oxysulfate are substantially insoluble.
In the Moa Bay process, lateritic ore at minus 20 mesh (95% passing 325 mesh U.S. Standard) is pulped to approximately 45% solids and the nickel and cobalt selectively leached with sufficient sulfuric acid at elevated temperature and pressure (e.g. 230.degree. C. to 250.degree. C. and 405 to 580 psia) to solubilize about 95% each of nickel and cobalt in about 60 to 90 minutes. After pressure let down, the leached pulp is washed by countercurrent decantation with the washed pulp going to tailings. The leach solution pH, which is quite low (e.g., between 0 and 0.5), is then neutralized with coral mud to a pH of about 2.4 in a series of four tanks at a total retention time of about 20 minutes and the thus-treated product liquor (containing about 5.65 gpl Ni, 0.8 gpl Fe and 2.3 gpl Al), after solid-liquid separation, is then subjected to sulfide precipitation. The leach liquor is preheated and the sulfide precipitation carried out using H.sub.2 S as the precipitating reagent in an autoclave at about 120.degree. C. (250.degree.F.) and a pressure of about 150 psig.
In the original scheme for treating the mixed sulfides, the sulfide precipitate was washed and thickened to a solids conter of 65%. It was then oxidized in an autoclave at about 177.degree. C. (350.degree. F.) and a pressure of about 700 psig.
The solution containing nickel and cobalt was then neutralized with ammonia to a pH (5.35) sufficient to precipitate any residual iron, aluminum, and chromium present using air as an oxidizing agent.
The precipitate was thereafter separated from the solution and the nickel and cobalt solution then adjusted to a pH of about 1.5. H.sub.2 S was added to precipitate selectively any copper lead and zinc present. The precipitate was separated from the solution by filtration and the nickel recovered by various methods, one method comprised treating the nickel-containing solution with hydrogen at elevated temperature and pressure to produce nickel powder.
The aforementioned process is similar to that described in "the state of the prior art" set forth in U.S. Pat. No. 4,097,575, the disclosure of which is incorporated herein by reference.
Certain lateritic ores, in particular saprolite ores, generally have a high magnesium content and a relatively low iron content compared to limonite which must be contended with in order to efficiently recover the nickel from the pregnant leach liquor and to separate the nickel from iron, magnesium and other impurities.
Commercial practice is to smelt high grade saprolitic ore containing generally in excess of about 2% nickel to produce either ferro-nickel or nickel matte.
With respect to limonite, the nickel is extracted from the ore by high pressure leaching using sulfuric acid as the lixiviant and/or reduction roasting followed by ammonia leaching.
Acid leaching of saprolitic ore is not practiced commercially for the reason that a process has not been developed for recovering the nickel from the leach solution in an economical and simple manner.
A typical high magnesium and high iron laterite generally contains by weight at least about 5% magnesium, for example, such as 10% and higher.
The Moa Bay process would not be suitable for treating such ores due to the excessive consumption of sulfuric acid because of the high magnesium content as MgO in the ore.
We have discovered a method for leaching laterites of the saprolitic type under ambient pressure and temperature, e.g., room temperature for column/heap leaching and about 60.degree. C. to 80.degree. C. for agitation leaching, wherein the by-product oxides of magnesium and iron can be used to good advantage for recycle within the leaching system as a means of controlling the pH of the pregnant nickel solution prior to the extracting of nickel from the solution.